Recent and predicted exponentially increasing demand for higher data rates in cellular radio communication networks sets new challenges to wireless network operators and equipment vendors. A question for operators is how to evolve their existing cellular networks in cost- and time-efficient manners so as to meet the demand for higher data rates. Network operators can choose among a number of possible approaches, including increasing the density of their existing base stations, increasing cooperation among base stations, and deploying smaller base stations in areas where high data rates are needed within a grid, or layer, of larger, or “macro”, base stations.
The last option can be called a heterogeneous network or heterogeneous network (HetNet) deployment. The network layer including the larger base stations can be called the macro layer, and the network layer including the smaller base stations can be called a “micro” or “pica” or “femto” layer. For example, the width of a macrocell can be greater than about two kilometers, the width of a microcell can be less than about two kilometers, the width of a picocell can be less than about two hundred meters, and the width of a femtocell can be a few dozen meters, but the artisan will understand that different widths in these ranges can be used. Thus, a HetNet in general has a mixture of cells, or base stations, of differently sized and overlapping coverage areas.
Improved support for heterogeneous cellular communication network operations is part of the ongoing specification of a Long Term Evolution (LTE) communication system by the Third Generation Partnership Project (3GPP) in its Release 10 Technical Specifications (TS) and in other upcoming Releases. 3GPP technical specifications for LTE networks can be seen as an evolution of the technical specifications for current wideband code division multiple access (WCDMA) networks. An LTE network is sometimes also called an Evolved Universal Terrestrial Radio Access (E-UTRA) Network (E-UTRAN).
FIG. 1 depicts an example of a HetNet 100 that includes three non-overlapping micro/pico/femto cells 110, 112, 114 deployed within the coverage area of a macro cell 120. It will be understood that the network 100 typically includes more than one macro cell 120, each of which can have zero, one, or more micro/pico/femto cells. In general, there is a significant difference in transmitted output power between a macro cell (e.g., +46 dBm) and a micro/pico/femto cell (e.g., less than +30 dBm). Examples of micro/pico/femto cells and similar low-power nodes in HetNets are home base stations and relay nodes. Base stations can also be called radio access network (RAN) nodes.
Building a denser layer of macro base stations and increasing cooperation between them can in principle meet current and future demand for higher data rates, but doing so is not necessarily either cost- or time-efficient because of the costs and delays involved in installation of macro base stations, especially in urban areas. As a result, deploying small, low-power base stations within an existing macro layer can be a more appealing option for a network operator, since micro/pico/femto base stations can be expected to be cheaper than macro base stations and the time needed to deploy them can be expected to be shorter.
Nevertheless, a dense deployment of low-power base stations can lead to a significantly higher amount of signaling overhead, with resulting reduced network capacity, due to more frequent cell-to-cell handovers of moving user equipments (UEs), which in general can be any type of wireless device or terminal, such as a telephone, laptop or tablet computer, a modem, a router, etc. The macro layer of a network, whether a HetNet or a homogeneous network, can serve UEs moving at high speed, and can also serve wider areas where the demand for high data rates is less. In a HetNet deployment, the smaller base stations can serve areas having a higher density of users requiring high data rates. Such areas are sometimes called “hotspots”.
As noted above, one goal of the low-power RAN nodes in a HetNet is to absorb as many users as possible from the macro layer, thereby reducing the load on the macro layer and enabling higher data rates in both the macro and micro/pico/femto layers. In addition, a UE can generally be expected to have better radio performance, especially in the uplink (UL) from the UE to the base station, when the UE is connected to a micro/pico/femto cell since the UE is likely to be closer to the small base station.
Two techniques that are used for enhancing cellular networks are extending the communication range of a RAN node by using cell-specific cell-selection offsets, and increasing the transmit power of a RAN node and simultaneously setting appropriate UL power control target values for UEs connected to RAN nodes. These techniques can be used in homogeneous network and HetNet deployments, but both techniques have a drawback in that interference in the downlink (DL) control channels from base stations to UEs increases. Since DL control channels may be transmitted over the whole network bandwidth, the usual inter-cell interference coordination (ICIC) mechanisms specified in 3GPP Releases 8 and 9 cannot be applied to them.
With ICIC techniques according to 3GPP Release 10 specifications, radio resources on a carrier are shared by coordinating transmissions between neighboring cells. In a HetNet deployment, for example, certain radio resources are allocated to a macro cell during certain time periods, thereby enabling remaining radio resources to be used by underlying micro/pico/femto cell(s) without interference from the macro cell. This kind of resource sharing can change over time to accommodate different traffic demands and traffic situations between cells or across network layers, and can be more or less dynamic, depending on the implementation of the interface between the cells, or network nodes.
In an LTE network, for example, base stations, or evolved NodeBs (eNBs), can communicate with each other via an X2 interface, and so an eNB can readily inform other eNBs that it will reduce its transmit power on certain radio resources. Messaging according to the X2 protocol is specified in 3GPP TS 36.423 v10.0.0, Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 application protocol (X2AP) (Release 10) (December 2010) and other specifications. Time synchronization of the eNBs is required to ensure that ICIC works efficiently, and this is particularly important for time-domain-based ICIC schemes, in which radio resources are shared in time on the same carrier.
A technique that has been investigated in 3GPP as an evolved ICIC mechanism, especially for the DL physical layer control channels, is the use of almost blank subframes (ABS). For example, a HetNet can use ABS for open-access micro/pico/femto eNBs that are closed subscriber group (CSG) home eNBs (HeNBs). With ABS in a HetNet, the macro layer is muted so as not to create high other-cell interference to UEs either that are connected to a low-power micro/pico/femto RAN node and located near the range limit of the low-power RAN node, or that are connected to a macro RAN node and located near a HeNB that does not belong to a CSG.
Nevertheless, ABS has a drawback in that radio resources are not fully used in some cells. For example in a HetNet deployment with a macro cell that is heavily loaded and a micro/pico/femto cell that has a low number of UEs located at the micro/pico/femto cell's range limit, a number of UEs connected to the macro cell will have to underutilize their radio resources so as not to interfere with the UEs in the micro/pico/femto cell. This inefficient use of radio resources can become even more pronounced if the micro/pico/femto cell UEs cannot receive DL control signaling, or they suffer high interference on the data regions of their DL signals due to cell-specific reference symbols (CRS) transmitted by the macro cell. The scenario is similar for UEs connected to macro eNBs and located near CSG HeNBs, as such UEs either cannot receive DL control signaling or suffer high interference on their data regions due to CRS transmitted by the near CSG HeNBs.
For efficient operation, LTE will require that transmissions from cells participating in ABS are time-aligned at the level of an orthogonal frequency division multiplex (OFDM) symbol. The starting OFDM symbols of transmission time intervals (TTI) either can be aligned between the macro and micro/pico/femto layers, or transmissions can be shifted in time in a multiple of OFDM symbol durations. In both cases, either the DL control channel region of the micro/pico/femto layer, or the data region, or both are going to receive strong interference by the CRS of the macro layer. Thus, whether UEs compliant with 3GPP LTE Release 11 will have to support interference cancellation of other cells' CRS is under discussion.
A number of different algorithms for setting the cell selection offset are known in the literature, which includes International Application PCT/SE2011/050604 filed on May 12, 2011, for “Methods in Base Stations, Computer Programs and Computer Program Products”, and International Application PCT/EP2011/051050 filed on Jan. 26, 2011 for “A Method and a Network Node for Determining an Offset for Selection of a Cell of a First Radio Network Node”.
Many algorithms for setting the cell selection offset are based on i) the ratio of the received power of reference symbols from a serving cell and a neighboring cell, which can be a micro/pico/femto cell, ii) the load on a macro cell or layer, iii) the load on a micro/pico/femto cell or layer, iv) the distances of macro base stations, etc. It seems likely, however, that such algorithms will not operate appropriately because UEs selected to be handed over to a neighbor cell, such as a micro/pico/femto base station, may not be able to operate when located at the range limit of the small cell. For example, some UEs in some cases may be taken up by a neighbor cell and be located in an extended range area of the neighbor cell without being able to receive DL control information from their serving cell.